Several arc welding methods are employed for consumable electrodes wherein the electrode is melted by current flow through the electrode, across an arc and to a workpiece. Such welding processes are divided between transfer of the molten metal from the consumable electrode to the workpiece by a surface tension, short circuit transfer mechanism or a non-short circuit transfer mechanism. In molten metal transfer of the type not involving a short circuit component, the molten metal on the end of the electrode is transferred across the electric arc to the weld puddle by way of electromagnetic forces. In such processes, the consumable if electrode should not contact the molten metal puddle constituting the workpiece. In these non-short circuit metal transfer processes for electric arc welding, a common procedure is either a spray transfer or pulse transfer. Since spray transfer requires a substantial amount of energy and high heat input at the molten metal puddle, in electric arc welding of the non-shorting type when lower heat is desirable, a pulse welding process is used. The pulse process utilizes lower heat to generate a less fluid molten metal puddle on the workpiece. This facilitates out of position welding and improves various mechanical aspects of the welding process.
The present invention relates to the implementation of a pulse welding process which improves the welding characteristics. In the past, a high current pulse was applied across the arc between the electrode and the workpiece at least when the molten metal droplet is formed on the end of the electrode. This high current pulse causes the droplet or molten metal mass on the end of the electrode to separate from the electrode by an electric pinch action, after which the molten metal mass or droplet is propelled across the arc to the molten metal puddle constituting the workpiece. The energy in the current pulse used for separating and propelling the molten metal to the workpiece is an important parameter of the overall welding process. The electric pinch action exerted on the droplet to constrict and separate the droplet from the electrode is proportional to the square of the applied current during the current pulse. Therefore, it would appear that greater applied current during the separation of the molten metal would result in a more rapid separation of the droplet for transfer to the workpiece and consequently a superior welding process. However, current flowing through the arc during the welding process exerts a magnetic force on the molten puddle, pushing the puddle downwardly away from the end of the consumable electrode. Such downward force on the molten metal in the puddle pushes the molten metal outwardly and results in a puddle depression below the electrode. This depression and the associated electromagnetic forces can cause extreme weld puddle agitation, especially when welding metals having low specific gravity, such as aluminum. The high energy created puddle agitation produces a poor appearance for the weld bead and unduly deep penetration of the metal into the workpiece being welded. Consequently, the parameters of the current pulse necessary to create an effective electric pinch of the droplet from the electrode must be accurately controlled to optimize the pinch action, but also to minimize the puddle agitation. The magnitude and shape of the current pulse used in the pulse welding process accomplishes this objective by reducing the amount of energy so that the pulse results in a smooth metal transfer and a minimum puddle agitation. The amount of energy in the current pulse is important; however, it is generally compromised to optimize diverse requirements of the welding process.
Most power supplies used for electric arc welding in the pulse welding mode have a variety of individual controls for adjusting the shape of the current pulse and/or the rate of the current pulse. Adjustment of the power supply to obtain a single droplet for each current pulse produces a very desirable performance. If the current pulse does not contain sufficient energy, the electrode does not melt and separate before the moving electrode engages the weld puddle to create an inadvertent undesirable short circuit. When this occurs, a substantial amount of spatter is generated in the welding process. Thus, the electric pulse must have a certain amount of energy to allow efficient metal transfer and avoid spatter creating, short circuit conditions. However, if the energy in the current pulse is too great, severe puddle agitation occurs as previously described and the molten metal on the end of the workpiece may be remelted to produce enlarged molten metal balls. This phenomena usually causes a premature shorting during the current pulse in a subsequent weld cycle. Due to these characteristics of a pulse welding process, the electric current pulse is extended to assure transfer even though such current pulses are too lengthy and cause puddle agitation.